"What we say is that the measures, the controls, are legal and proportionate. We are obligated to do them under Schengen. We can't renounce them," a foreign ministry spokesman said in reference to the EU's borderless travel zone, which Britain does not belong to.

Also sailing tomorrow will be HMS Westminster, a type 23 frigate, which will visit Gibraltar en route.

Other UK ships taking part are another type 23 frigate, HMS Montrose, and six Royal Fleet Auxiliary (RFA) ships.

Madrid has agreed to allow HMS Illustrious to stop at a naval base in Rota in southern Spain as part of the operation, but Spanish media have described the plan for HMS Westminster to put in at Gibraltar as an intimidation.

A cloud is seen over the Rock of Gibraltar from a parking lot next to the border where drivers leave their vehicles to commute by foot into Gibraltar

Britain and Spain are embroiled in an escalating diplomatic row over stringent car searches by Spanish guards at the Gibraltar border, causing tailbacks of several hours.

The government of the tiny peninsula has accused Madrid of acting in retaliation after Gibraltar built an artificial concrete reef which it says is aimed at stopping alleged incursions by Spanish fishing boats.

Spain is considering taking its claims to global bodies such as the United Nations and International Court of Justice at The Hague, a foreign ministry spokesman said on Sunday.

Spain could also team up with Argentina, which is on a two-year term as non-permanent member of the UN's Security Council, to address the issue of the sovereignty of Gibraltar and the Falkland Islands, or the Malvinas as they are known in Spanish, in international bodies, he added.